If a position of an occupant can be detected around a vehicle or in a vehicle interior, it is considered that various services can be provided to the occupant. Therefore, today, a technique for detecting a position of a mobile terminal (therefore, the position of an occupant) by installing small receivers at multiple places of a vehicle and receiving radio waves from the mobile terminal has been proposed focusing on the fact that a large number of occupants carry a mobile terminal capable of performing a wireless communication.